The invention is embodied in a head up display unit to filter out solar radiation capable of washing out the display information.
Typically, in a head up display (hereinafter also referred to as a HUD) of the type exemplified by the description in U.S. Pat. No. 3,940,204 issued on Feb. 24, 1976 to R. J. Withrington et al and entitled "Optical Display Systems Utilizing Holographic Lenses", an observer in a cockpit looks through a holographic diffraction grating lens at a scene. An image of display information from a source, such as a cathode ray tube located in the aircraft out of the line of sight, is combined with and superimposed on the scene by means of an optics system which includes relay optics, a folding reflection and the holographic lens. In operation, the optics system directs the image of the display information to the holographic lens where it is diffracted or reflected from the holograph lens to the viewer's eyes whereupon it is effectively combined with the scene. As a result of this combining function the holographic lens will hereinafter also be referred to as a combiner.
Head up displays of this type can be subjected to solar noise in the form of solar radiation which, when the sun is within a range of certain critical incident angles, will pass back down the optical path to wash out the display information on the cathode ray tube.
A number of possible solutions to this solar noise problem have been investigated, but most work effectively only over a limited angular range of incident angles of incoming radiation. Examples are narrowband reflection or transmission filters. A typical narrowband transmission filter is made of a high index of refraction material, typically thin layers of metals and dielectrics, which provides a narrow bandpass with high transmittance. This type of filter is preferably tilted across the optical axis in front of the relay optics with the cathode ray tube being positioned in back of the relay optics. In operation, this filter reduces the amount of solar radiation which is directed to the face of the cathode ray tube over a limited range of incoming sun ray angles.
Another approach is to use a narrowband reflecting mirror which operates somewhat similarly to the tilted transmission filter but in a reflection rather than a transmission mode. Often the mirror includes a layer of absorption glass positioned before the dichroic mirror reflection surface. Again the disadvantage of this approach is the limited angular range of effectiveness.
Other solutions which have been investigated but which have not proven successful are a moving mechanical blocking apparatus and a venetian blind type of blocking element.